1. Field of the Invention
The instant disclosure relates to an LED backlight driving module, in particular, the invention relates to a LED backlight driving module having a uniform current output in each LED light bar.
2. Description of Related Art
In recent years, LED backlight is being increasingly applied to the fabrication of large-size LCD display apparatus. Furthermore, the LED backlight provides a solution to environmental protection and power saving since the LED devices do not contain heavy metals such as mercury. Relevant statistics shows that the LED backlight technology is rapidly replacing traditional backlight units with a growing market penetration rate from 3% in 2009 to 20% in 2010.
Currently, LED backlight units can be categorized into direct type and edge type. The direct-type LED backlight units have the advantage of partial light dimming capability to comply with specific video requirements, thus achieving better performance in power-consumption, higher dynamic contrast, and greater color domain. However, the direct-type LED backlight requires more LED units and more complicated driving modules. The edge-type LED backlight, on the other hand, excels in its adaptability in thinner LCD displays. The edge-type LED backlight also requires fewer LED units and less complicated driving modules, thus has a lower production cost. However, the edge-type LED backlight can not be performed two-dimensional partial dimming, and has a poorer dynamic contrast.
The driving modules of the conventional edge-type LED backlight units can be divided into DC-DC current conversion circuits, as shown in FIG. 1, or linear current regulator and PWM dimming circuits, as shown in FIG. 2. The driving module is employed to uniformly drive a plurality of LED light bars.
Reference is made to FIG. 1, which shows a driving module 10 of a conventional LED backlight. A power isolation transformer 12 of a power circuit 19 is incorporated into the driving module 10. The power isolation transformer 12 performs a voltage conversion on a signal from a main power stage 11. An output rectifier and filter 13 rectifies and filters the power stage signal, generating a driving power signal. A plurality of DC-DC converters (#1, #2, #3 . . . #N) 16 subsequently performs a second voltage conversion on the driving power signals. The converted signal is then adapted to drive a plurality of LED light bars (#1, #2, #3 . . . #N) 17 to produce light. A plurality of PWM dimming and current feedback circuits (#1, #2, #3 . . . #N) 18 are incorporated to modulate the luminance of the LED units and to stabilize the LED current. A feedback circuit 14 and a PWM controller 15 are adapted to provide feedback control, thus stabilizing the driving power signal of the power circuit 19.
The cost and area used by circuit board will increase when the number of the LED light bars 17 increases and the corresponding DC-DC converters 16 need to reach the same number as well. Furthermore, interference might occur if the operation frequency of the DC-DC converter 16 is not synchronized with the main power stage 11. Still further, the DC-DC converter 16 generates additional electromagnetic radiation interference (EMI).
Reference is now made to FIG. 2, which shows the topology of a conventional driving module 20 similar to the module 10 of FIG. 1. Module 20 uses a plurality of linear current regulator and PWM dimming circuits (#1, #2, #3 . . . #N) 21 to replace the DC-DC converter 16 and the feedback circuit 18.
Furthermore, when the individual differences among the plural LED light bars 17 are large, the extra loss of the system would also increase because the driving module 20 uses the linear current regulator and PWM dimming circuit 21. Yet further, if the number of LED dies in-series connected in one LED light bar 17 increases, the power consumption and waste heat of each linear current regulator and PWM dimming circuit 21 also increase. Thus, additional cooling fins would be needed to maintain normal operation. Therefore, the number of the connected LED dies requires an upper limit in order to prevent over-heating and failure of the linear current regulator and PWM dimming circuit 21, and allows the cooling fins to have a best cooling efficiency.